1. Field of the Invention
The present invention relates to a semiconductor light-emitting device using a light-emitting diode element.
2. Description of Related Art
Conventionally, in semiconductor light-emitting devices of surface mount type, it has been significant challenges to improve emission efficiency for the sake of extended battery life of intended apparatuses, as well as to achieve a further miniaturization. For improved emission efficiency, some devices are configured so that the periphery of light-emitting diode (hereinafter, abbreviated as LED) element, excluding its light-emitting surface, is covered with a white resin which has a high light reflectance and diffuse reflection effect. (For example, see Japanese Patent Application Laid-Open No. 2005-277227).
On the other hand, conventional light-emitting devices have left room for improvement regarding the effective use of light that is laterally and downwards emitted from a junction of the LED element.
In order to efficiently use light that is emitted from LED element downwards, many semiconductor light-emitting devices have a printed-wiring board that is plated with silver or the like to increase reflectance at the component side for the LED element to be mounted on. Since the LED element emits light radially from its junction, it is difficult to use downward light emitted from its junction efficiently unless the silver plating has a sufficient area. This has left a problem of miniaturization.
FIG. 8 shows a conventional semiconductor light-emitting device 70 which incorporates an LED element 60. The reference numeral 71 represents a printed-wiring board on which the LED element 60 is mounted. A pair of substrate electrodes 72 and 73 is formed on the top of this printed-wiring board 71 so as to wrap around both sides. A reflective film 74 made of metal with a high light reflectance, such as aluminum and silver, is formed on the surface of one substrate electrode 73. Moreover, two element electrodes 54 and 55 of the LED element 60 are connected to the substrate electrodes 72 and 73 of the printed-wiring board 71 by wires 75, respectively. The LED element 60 is sealed with a transparent or translucent resin body 76. The outer periphery except a surface contact with an upper surface of the printed-wiring board of this resin body 76 is configured to be light-emitting surfaces 77.
In the LED element 60, its PN-junction 53 emits light of high intensity radially. Of the light emitted, emission light 78 that travels downward at steep angles from the PN-junction 53 is reflected by the reflective film 74, and is thus directed upward with relatively high efficiency. Emission light 79 that travels slightly downwards from the PN-junction 53 is reflected by the substrate electrode 73 outside the reflective film 74. Also, light emitted laterally and downwards without being reflected, refracts in further downward directions when being emitted out of the resin body 76, and thus, the intensity of light emitted out of the light-emitting device 70 has room for improvement.
In view of bondability of the wires 75, it is desirable to plate the surfaces of the substrate electrodes 72 and 73 with gold. The gold plating, however, has the problem of extremely low reflectance particularly for blue LEDs. As mentioned above, since the PN-junction 53 emits light of high intensity also downward, there has been a significant problem with emission efficiency in that much of light travels laterally downward like the emission light 79.
FIG. 9 shows a semiconductor light-emitting device for solving the foregoing problem, in which reflective films 74a and 74b are provided on the entire surfaces of the substrate electrodes 72 and 73 formed on the printed-wiring board 71. Since the reflective films 74a and 74b are provided on the entire surfaces of the substrate electrodes 72 and 73, both the emission light beams 78 and 79 are reflected by the reflective films 74a and 74b, thereby solving the problem of emission efficiency. However, provision of reflective films 74a and 74b such as aluminum and silver on the wire bonding surfaces can lower wire bondability and deteriorate reliability.
FIG. 10 shows a semiconductor light-emitting device 90 for solving the problem in the reliability of the wire bonding, wherein the width of the semiconductor light-emitting device 90 is increased to widen the reflective film 74. More specifically, the semiconductor light-emitting device 90 shown in FIG. 10 is given a width L2 greater than the width L1 of the semiconductor light-emitting device 70 shown in FIG. 8 (L1<L2). The width of the reflective film 74 is also increased accordingly. This configuration improves emission efficiency and enhances reliability of the wire bonding, whereas it counteracts the miniaturization.